The application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to a Japanese Patent Application 2002-082518, filed on Mar. 25, 2002, the entire content of which is incorporated herein by reference.
This invention generally relates to a passively modelocked fiber laser capable of providing an extra-short pulse, and more particularly, this invention pertains to a fiber isolator which removes a return laser beam returning from a cavity part to a laser diode.
A typical passively modelocked fiber laser is shown in FIG. 3. The passively modelocked fiber laser is mainly composed of a pump part 20 and a cavity part 30. The pump part 20 includes a laser diode 21 which generates a laser beam (laser energy) with a wavelength of 980 nm as an excitation light, a fiber isolator 22, and a single mode fiber 23 which propagates the laser beam generated at the laser diode 21 to the fiber isolator 22. The cavity part 30 includes a wavelength splitter 31 which guides the laser beam with the wavelength of 980 nm from the pump part 20, an erbium doped fiber 32 (a gaining medium) which amplifies the laser beam with a wavelength of 1560 nm by the excitation light having the wavelength of 980 nm, and a pair of metallic mirrors 33 and 34 (a reflecting means) which reflects a laser beam in the cavity part 30.
The cavity part 30 further includes collimator optical systems 35 and 36 which form an optical path between the metallic mirrors 33 and 34, a beam splitter 37 (an output means) which outputs the laser beam with the wavelength of 1560 nm, and a single mode fiber 38 which propagates the laser beam with the wavelength of 1560 nm between the wavelength splitter 31 and the collimator optical system 36. Propagation between the pump part 20 and the cavity part 30 is performed by a single mode fiber 24.
The laser beam with a wavelength of 980 nm generated at the laser diode 21 is propagated as single mode propagation because of the single mode fibers 23, 24, and 38 propagating a laser beam with a cutoff wavelength or larger than the cutoff wavelength. The laser beam guided by the wavelength splitter 31 is amplified to the laser beam with the wavelength of 1560 nm at the erbium doped fiber 32.
Part of a return laser beam (1560 nm) which returns to the fiber isolator 22 via the single mode fiber 24 from the cavity part 30 is discharged out of an optical fiber (not shown) which is part of the fiber isolator 22, or total internal reflection of the return laser beam (1560 nm) within the optical fiber is not generated and the return laser beam leaks outside. Consequently, the return laser beam incident on the laser diode 21 and with a wavelength of 1560 nm can be prevented by an inexpensive device as well as a downsized device.
However, in the conventional fiber isolator 22, all of the return laser beam is not discharged from the optical fiber, and part of the return laser beam returns to a core of the fiber within isolator 22 by being reflected on a boundary part between the optical fiber and the outside. Additionally, part of a laser beam once discharged outside is affected by a wavelength absorption characteristic of a coating, or some other apparatus, located on or around the optical fiber. For example, protected by a white coated tube, the laser beam leaking from the optical fiber is reflected by the white coated tube, returns to the inside of the optical fiber again, and is incident on the laser diode 21. When a return laser beam returning to a fiber isolator is incident on a laser diode, the following problems may arise; unsteady oscillation of a laser beam at a laser diode, dropping of signal-to-noise (S/N) ratio of a laser beam which has to be guided, or deterioration of the M2 factor. The M2 factor is an international standard for evaluating quality of a laser beam quantitatively.
The present invention, therefore, seeks to provide an improved passively mode locked fiber laser capable of solving the foregoing problems, for example, unsteady oscillation of a laser beam, the reduction of S/N ratio, and the deterioration of the M2 factor, when a return laser beam is incident on a laser diode.
According to an aspect of the present invention, a passively modelocked fiber laser includes a laser diode generating laser energy, a wavelength splitter guiding the laser energy generated at the laser diode to a cavity part, and a gaining medium for amplifying the laser energy in the cavity part. The passively modelocked fiber laser further includes a reflecting means, such as the mirrors shown in FIG. 3, for reflecting the laser energy along an optical axis passing through the gaining medium, an output means, such as the beam splitter 37 shown in FIG. 3, for outputting the laser energy generated in the cavity part, and a fiber isolator provided between the laser diode and the wavelength splitter. In the foregoing passively modelocked fiber laser according to the invention, the fiber isolator has an optical fiber having a curved shape. The optical fiber includes a core for propagating the laser energy, a clad for coating the core, and an optical absorption film for coating the clad.
According to another aspect of the present invention, the optical fiber has a coiled shape, and a value of a refractive index of the optical absorption film is equal to or greater than a value of a refractive index of the clad.